In-series application of reaction-based energy systems [e.g., batteries, fuel cells, and microbial fuel cells (MFCs)] can be limited by a phenomenon called voltage reversal. Voltage reversal occurs, for instance, when multiple batteries are arranged in series and one battery malfunctions due to reduced power capability while the other batteries are actively providing electric potentials. Under such a circumstance, electrode reactions in the malfunctioning battery are governed by the electric potentials of the adjacent batteries. As such, the anode of the malfunctioning battery is governed by a cathode potential from an adjacent battery and thus becomes more positive than the cathode of the malfunctioning battery which is dominated by an active anode of an adjacent battery. As a result, reduction occurs at the malfunctioning anode and battery materials are oxidized at the malfunctioning cathode. This “voltage reversal” not only prevents stable operation of the multiple battery system, but can also damage electrode systems of the malfunctioning battery.
Voltage generation from commercial batteries or fuel cells is generally less vulnerable against voltage reversal problems because their electrode reactions are chemically driven and therefore very stable. However, if electrode reactions are driven biologically by microorganisms, an in-series system can be more affected by voltage reversal as reported for MFCs (e.g. see Aelterman et al. 2006, Oh and Logan 2007). An MFC is a bioelectrochemical system for harvesting electric energy from aqueous organic matter. Exoelectrogenic bacteria at an MFC anode oxidize organic matter and simultaneously transfer electrons to the anode. A typical counter reaction at an MFC cathode is reduction of oxygen. This red-ox couple (i.e., oxygen reduction and oxidation of organic matter) creates a maximum theoretic voltage of 1.1 V (12-mM acetate as organic matter; pO2=0.2 atm; pH=7) (Logan 2008), however open circuit potentials of a well-controlled laboratory MFC usually do not exceed 0.8 V.
While MFCs can be used to convert organic contaminants in wastewater into electric energy, the low voltage problems have been one limiting factor for practical applications in large scale wastewater treatment plants. Attempts have been made to overcome the low voltage problems, but results have been unsuccessful. For example, multiple MFCs were arranged in series to increase output voltages; however, voltage reversal was easily induced by an imbalance in organic matter concentration (Oh and Logan 2007) and by a high current condition (Aelterman et al. 2006). Thus, in-series operation of MFCs has heretofore not been an option for practical applications of MFCs.